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• New approach of a system- and station-dependent TOPEX/Poseidon SLR measurement correction function. • It is based on normal point observations and improves the mean orbit RMS from 33 cm to 1.97 cm. • The correction function and the station correction parameters are publicly available. The TOPEX/Poseidon (T/P) altimetry mission with its main objectives to monitor variations of the global and regional sea level as well as ocean circulation is a milestone in Earth observation. The spacecraft was launched in 1992 and it is the predecessor mission of the Jason series and Sentinel-6A. For laser ranging measurements from the Earth to the spacecraft, T/P was equipped with a non-ideally designed annular retroreflector array. Its large dimensions of over 160 cm in diameter caused huge optical phase centre variations which limit the orbit accuracy. In this study, we developed a continuous, analytical correction function that counteracts both, phase centre variations at the spacecraft and station-related laser ranging measurement errors such as range biases. For most Satellite Laser Ranging (SLR) ground stations that tracked the T/P spacecraft, an individual set of six correction parameters is estimated. The parameters are valid for the entire mission or, in some cases, for a defined period. The developed function uses the observation's viewing angles to determine a correction value which is added to the range measurement. Applying the measurement correction reduces the overall T/P mission root mean square fit of SLR residuals from 33.78 cm to 1.97 cm (1.59 cm for SLR core stations). External orbits based on the joint analysis of two different space-geodetic techniques are used to validate the quality of our improved orbit solution. The comparisons show good agreement. The mean values of the radial, transverse, and normal components differ by 0.0 , - 0.1 , and - 0.2 cm, respectively. To investigate the impact of the corrected orbit on sea level computations, a single-satellite crossover analysis is performed. When using the corrected measurements, the standard deviation of crossover differences reduces from 16 cm to 6 cm. The computed coefficients of the measurement correction function are provided publicly and can be used in any software package to obtain T/P orbits. [ABSTRACT FROM AUTHOR]

For low Earth orbiting satellites, non-gravitational forces cause one of the largest perturbing accelerations. During a precise orbit determination (POD), the accurate modeling of the satellite-body attitude and solar panel orientation is important since the satellite's effective cross-sectional area is directly related to the perturbing acceleration. Moreover, the position of tracking instruments that are mounted on the satellite body are affected by the satellite attitude. For satellites like Jason-1/-2/-3, attitude information is available in two forms—as a so-called nominal yaw steering model and as observation-based (measured by star tracking cameras) quaternions of the spacecraft body orientation and rotation angles of the solar arrays. In this study, we have developed a preprocessing procedure for publicly available satellite attitude information. We computed orbits based on Satellite Laser Ranging (SLR) observations to the Jason satellites at an overall time interval of approximately 25 years, using each of the two satellite attitude representations. Based on the analysis of the orbits, we investigate the influence of using preprocessed observation-based attitude in contrast to using a nominal yaw steering model for the POD. About 75% of all orbital arcs calculated with the observation-based satellite attitude data result in a smaller root mean square (RMS) of residuals. More precisely, the resulting orbits show an improvement in the overall mission RMS of SLR observation residuals of 5.93% (Jason-1), 8.27% (Jason-2) and 4.51% (Jason-3) compared to the nominal attitude realization. Besides the satellite orbits, also the estimated station coordinates benefit from the refined attitude handling, that is, the station repeatability is clearly improved at the draconitic period. Moreover, altimetry analysis indicates a clear improvement of the single-satellite crossover differences (6%, 15%, and 16% reduction of the mean of absolute differences and 1.2%, 2.7%, and 1.3% of their standard deviations for Jason-1/-2/-3, respectively). On request, the preprocessed attitude data are available. [ABSTRACT FROM AUTHOR]

The location of the Earth's principal axes of inertia is a foundation for all the theories and solutions of its rotation, and thus has a broad effect on many fields, including astronomy, geodesy, and satellite-based positioning and navigation systems. That location is determined by the second-degree Stokes coefficients of the geopotential. Accurate solutions for those coefficients were limited to the stationary case for many years, but the situation improved with the accomplishment of Gravity Recovery and Climate Experiment (GRACE), and nowadays several solutions for the time-varying geopotential have been derived based on gravity and satellite laser ranging data, with time resolutions reaching one month or one week. Although those solutions are already accurate enough to compute the evolution of the Earth's axes of inertia along more than a decade, such an analysis has never been performed. In this paper, we present the first analysis of this problem, taking advantage of previous analytical derivations to simplify the computations and the estimation of the uncertainty of solutions. The results are rather striking, since the axes of inertia do not move around some mean position fixed to a given terrestrial reference frame in this period, but drift away from their initial location in a slow but clear and not negligible manner. [ABSTRACT FROM AUTHOR]